1.1. Soil – Air Transfer

So, now moving on from sediment, similar kind of system with one significant difference is soil-air interface. Now, soil-air interface is much closer to us because we have interface with it directly, sediment-water interface is somewhere down there. So, a lot of times, sediment water problems are not even seen until a several decades after it is done. Soil-air you can see it very quickly for two reasons. One, there is groundwater sitting right next to it. So, it may reach the groundwater quickly.

The soil-air interface represents a boundary where soil meets the air we breathe. This interface is critical because it is easily observable and has immediate relevance to us. Unlike sediment-water interfaces, which can take decades to reveal environmental problems, concerns at the soil-air interface manifest quickly. This is largely due to the proximity of groundwater, which can be contaminated by materials that evaporate from the soil.

Imagine you are playing in your backyard, and a neighbor spills some chemicals nearby. You can see the surface immediately. If this happened in a river nearby, it could take a long time before you notice any negative effects. This illustrates how soil-air interactions can be observed and addressed faster than issues beyond our immediate view.

So, let us say that there is a contamination here, why are we worried about soil-air exchange? If there is a contamination sitting here inside soil or there is a contaminations sitting right on top, so we did examples in the beginning of this mass transfer exercise that we said that there is impervious land surface. The chemical is dumped on the road or surface which is not porous. It sits there and it evaporates from the pure chemical from there, but if it is not impervious, it is soil, it percolates just the way chemicals into the sediment percolate over a period of time and then it reaches a state where pure chemical is either sitting there or it is all gone and then it is adsorbed, then it starts reversing, it starts going out.

Contamination in soil is concerning because it can easily transfer to the air and potentially contaminate groundwater. For example, if chemicals are dumped onto an impervious surface like asphalt, they may just evaporate. However, if the surface is soil, those chemicals can seep into the ground and may later evaporate back into the air, affecting the environment and human health. This makes understanding the mechanisms of soil-air exchange essential to address contamination.

Think of a sponge soaked with water. If you dump a dye into the water, you can see its immediate effect. If you try this with a solid surface instead, the dye simply might evaporate and disappear. Similarly, contaminants influence their surroundings in various ways depending on the medium, emphasizing the need for awareness of contamination in soils.

Everything we have done in the sediment case applies here, the model, we use the same model, this domain equation will still be it will just be 𝑞𝑟 instead of 𝑗. Instead of sediment-water interface, we are using soil-air interface...

The mathematical modeling used for studying sediment transfer can also be applied to soil-air transfer. The core equations need adaptations for the parameters specific to soil, like moisture content and chemical properties. For example, equations that describe the behavior of chemicals in sediment under water can also describe the diffusion of chemicals in soil as they move to the air, with some variations based on soil moisture.

Consider a recipe for making dough. The basic steps are similar whether you make bread or cookies, but you may need different amounts of ingredients or baking times. Similarly, while the principles governing diffusion remain the same, the application to soil requires consideration of the unique properties of soil.

Now, here you have to consider all the usual things that if the soil is wet the partition constant is based on certain properties... So, the retardation factor can change with time, this is something that happens in soil. So, 𝑞𝑟 is a function of the moisture and then moisture content is a function of time.

The moisture content in soil greatly influences how chemicals are transferred between soil and air. A wet soil may have a different partition constant than dry soil, affecting the speed and efficiency of chemical movement. Over time, variations in moisture can change the retardation factor, reflecting how quickly chemicals can move out of the soil.

Think of how a sponge filled with water holds the dye longer than a dry sponge. If the sponge is wet, the dye can spread quickly within it and may escape when squeezed, similar to contamination behaviors in wet versus dry soils.

So the flux n versus t, so let us say we start with a wet season... what happens by May? The soil starts drying very rapidly...

Flux in and out of soil is affected by seasonality; during wet months, chemicals can easily diffuse from the soil into the air, but as the soil dries, this ability diminishes. Consequently, the flux drops significantly and then may spike with new rainfall or humidity. This cyclical behavior reflects how soil and air conditions interact throughout seasons.

It's like a garden that flourishes in spring rain, with flowers blooming and scents floating in the air, but during a dry spell in summer, the flowers wilt, and fewer scents are released. This demonstrates how the environment around us affects how chemical emissions can vary throughout the year.

To get a correlation for mass transfer coefficients, you need to... you take a sample of air and analyze it by whatever techniques you already learned...

Measuring flux involves quantifying how much chemical moves through the soil and into the air over time. This requires careful sampling of air and soil to determine concentrations, using techniques learned in analytical chemistry. By applying the mass balance principle, scientists can derive the flux from measured concentrations in the systems studied.

Imagine filling up several balloons with air at different times and analyzing how much air each has. By measuring the volumes, you can understand how air is exchanged over time. Similarly, scientists apply sampling techniques to uncover flux dynamics in soil and air.

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